The use of test devices or strips in analysis and, in particular, in clinical chemistry belongs to the established processes. In comparison with conventional methods of wet chemistry, such analyses are faster and easier to carry out.
However, when test strips are used in analysis, especially in whole blood, it is frequently necessary to wipe the strip. This wiping operation can lead to damage to the edges and surfaces of the reagent pad projecting out from the plane of the substrate. Subsequent evaluation, for example with a reflectance photometer, is thereby influenced and can lead to results which cannot be reproduced.
Typically reagent test devices contain one or more reagent matrix areas or pads attached to a substrate. The matrix areas or pads extend above the surface of the substrate and accordingly these areas or pads are exposed and tend to be damaged during (a) transport, (b) removal of the test devices from their container, and/or (c) in use, e.g., when wiping a test device to remove red blood cells, contaminants, etc. Not only are the edges of the test devices damaged, but frequently the surface of a pad is damaged. Not uncommonly, the entire pad or reagent matrix area is "knocked off" of the substrate during the wiping operation, particularly if the wiping tends to be vigorous. This has been a problem of long-standing which has required the attention of all manufacturers of diagnostic test devices.
Exact positioning of the reagent pad or matrix on the substrate carrier is also important for evaluation of the test strips with a reflectance photometer. If a planar surface is present for the substrate, there is the risk that the reagent pad will be brought out of the exact position as a result of inadequate adhesion of the adhesive which binds the pad to the substrate.
Inadequate adhesion of the adhesive can exist, for example, if the adhesive used has not yet hardened completely during production and packaging, or the adhesive ages as a result of storage.
Different adhesives can be required in reagent strips containing multiple reagent pads attached to a common substrate in order to ensure compatibility of the reagents in each pad with the particular adhesive employed. Exact metering of the amount of adhesive presents difficulties and leads to indeterminate hardening of reagent strips.
The influence of the residual solvent in the adhesive on the storage life of and change in the reagents introduced into a reagent strip can also be a disadvantage.
Another disadvantage is that adhesives can migrate into the reagent pad or matrix in an uncontrolled manner. Absorption of a defined volume of sample by the reagent strip is thus no longer guaranteed, which can lead to erroneous analytical results.